Shaft induced electrical current is experienced in electric motors, and commonly in three-phase motors driven by variable speed drives. Variable speed drives utilize pulse width modulation technology to vary the speed of AC motors, thereby allowing use of less-expensive AC motors in applications where more expensive DC motors had been used previously. A drawback to the use of AC motors with variable speed drives is that higher common mode voltage (CMV) is generated by the variable speed drive, which increases shaft induced currents.
Voltage on the motor shaft induces current flow through the shaft bearings to the motor frame and then to ground. While the motor is running, the bearings become more resistive to current flow, causing a buildup of charge on the shaft surfaces. Over a short period of time, the CMV causes electrical charges to build to a high level. As the electrical charges pass the threshold level of the least electrically resistive path, sometimes through the ball bearings on the shaft, an instantaneous burst or discharge of electrical energy passes along the path. The discharge can cause electric discharge machining (EDM) along the path, which can damage the surfaces of the bearing races and the balls in the bearing if the least resistive path is through the bearings. The electrical energy burst creates fusion craters, and particulate from the crater formation remains inside sealed bearing. Both the fusion crater and the particulate material in the bearing act to disturb the free rotation of the bearing, which can lead to physical damage and premature bearing failure.
A number of mitigation technologies are known to have been used in attempts to overcome this problem. Known attempts include using conductive bearing grease, insulating the bearings and using copper/phosphorus brushes and a Faraday shield. A common, somewhat cost-effective solution is to ground the shaft using spring-loaded copper brushes that provide a continuous flow of current to ground. However, copper brushes can wear out rapidly, requiring frequent, periodic service and replacement. Additionally, oxide build-up on the shaft and other barriers between the brushes and the shaft reduce the current flow and cause a burst of electrical energy across the brush and shaft. Spring-loaded brushes also tend to vibrate due to alternating frictional relationships between the brush and the shaft surface. Vibration of the brushes, from whatever cause, can result in undesirable sparking.
It is known to use grounding brushes that include conductive filaments in a holder surrounding the shaft. The brush with thin filaments can be used as a non-contacting ionizer to reduce electrical charges on the isolated shaft or on an isolated roller. The thin, light filaments can also be used as a contacting conductor against a rotating shaft or other moving surface. However, the effectiveness of the thin filament grounding brushes either as a non-contacting ionizer or as a contacting conductor can be compromised by properties of the surface with which it interacts. Corrosion of the shaft or other surface can adversely affect performance. The grounding performance of a new motor can decrease over time with corrosion of the shaft, and retrofitting a grounding system of this type can be problematic if the motor shaft is corroded.
What is needed in the art is a grounding system that can be used effectively for a prolonged period of time under adverse conditions, and which can be installed as a retrofit or incorporated into new assemblies.